The study is aimed at the analysis of the control of gene expression during the transition of proliferating precursor cells to postmitotic, terminally differentiated muscle cells. Myogenic cell lines, primary cultures, and cell lines derived from animals with hereditary diseases affecting the muscle tissue are being applied in this study. Recombinant plasmids containing cDNA sequences homologous to muscle-specific mRNA were constructed and applied as probes to follow gene expression at the RNA level and to screen rat genomic libraries for the corresponding genes. Recombinant phages containing DNA inserts homologous to myosin heavy chain, myosin light chain 2 and actin genes were isolated. Their structure and organization is being investigated.